Effect of the his residue on the cyclization of b ions

Abstract

The MSn spectra of the [M + H]+ and b5 peaks derived from the peptides HAAAAA, AHAAAA, AAHAAA, AAAHAA, and AAAAHA have been measured, as have the spectra of the b4 ions derived from the first four peptides. The MS2 spectra of the [M + H]+ ions show a substantial series of bn ions with enhanced cleavage at the amide bond C-terminal to His and substantial cleavage at the amide bond N-terminal to His (when there are at least two residues N-terminal to the His residue). There is compelling experimental and theoretical evidence for formation of nondirect sequence ions via cyclization/reopening chemistry in the CID spectra of the b tons when the His residue is near the C-terminus. The experimental evidence is less clear for ions when the His residue is near the N-terminus, although this may be due to the use of multiple alanine residues in the peptide making identifying scrambled peaks more difficult. The product ion mass spectra of the b4 and b5 ions from these isomeric peptides with cyclically permuted amino acid sequences are similar, but also show clear differences. This indicates less active cyclization/reopening followed by fragmentation of common structures for bn ions containing His than for sequences of solely aliphatic residues. Despite more energetically favorable cyclization barriers for the b5 structures, the b4 ions experimental data show more clear evidence of cyclization and sequence scrambling before fragmentation. For both b4 and b5 the energetically most favored structure is a macrocyclic isomer protonated at the His side chain.